Federal Reserve Bank of Atlanta President Dennis Lockhart said Wednesday that if what appears to be a weather-related slowdown in the U.S. economy proves more enduring than expected, he may push back the date when he first expects the central bank to raise short-term interest rates.

“Based on my working medium-term outlook, I see the latter half of 2015 as the likely time frame for the first move to higher rates,” Mr. Lockhart said. But if the economy doesn’t grow as he current expects, the central banker said “a later liftoff date than I am assuming will likely be appropriate.”

Mr. Lockhart’s comments came from the text of a speech to delivered in Miami before a local group. The official spoke amid a time of uncertainty about the outlook for the U.S. economy. Dreadful winter weather appears to have weighed heavily on U.S. growth over the start of the year, in the view of many central bankers. While most officials expect the impact of this weather to be transitory, if the weakness were to prove more enduring, it could cause the central bank to rethink the future path of monetary policy.

Most Fed officials believe, in a view shared by Mr. Lockhart, that gains in growth and inflation, joined with declines in unemployment, will allow the central bank to raise short-term rates from their current near-zero percent levels some time in 2015.

Mr. Lockhart is widely viewed as a bellwether for the monetary policy setting Federal Open Market Committee, although he isn’t currently a voting member of that group. His warning that the outlook for rates may be in flux may signal other central bankers are growing a bit anxious over the outlook.

The central banker said he believes it will be known “relatively soon” whether the weather’s negative influence is still in play. “My key working assumption is that growth will accelerate in the second quarter and repeat in subsequent quarters,” Mr. Lockhart said, adding that he and economists at his bank “are assuming a resumption of growth approaching 3% in the second quarter and beyond.”

Mr. Lockhart noted that growth is important to central bankers in as much as it leads to lower rates of unemployment and inflation moving back to the Fed’s 2% target level. While unemployment, currently at 6.7%, has declined faster than Fed officials expected, inflation has remained stubbornly weak and has in fact softened at a time when most policymakers have expected it to rise.

“I believe the FOMC is still significantly short of achieving its two mandated objectives,” Mr. Lockhart said. Because of this, “monetary policy should remain quite accommodative for some time.”

Mr. Lockhart also addressed the fate of the Fed’s ongoing bond-buying stimulus campaign, which currently sees the Fed buying $55 billion a month in Treasury and mortgage securities.

“I support the continued phase-out of the asset purchase program,” the official said. The program “is being wound down,” Mr. Lockhart said, noting “at this pace, purchases will drop to zero in October or December.”

About Real Time Economics

Real Time Economics offers exclusive news, analysis and commentary on the U.S. and global economy, central bank policy and economics. Send news items, comments and questions to the editors and reporters below or email realtimeeconomics@wsj.com.